fix(fuse): clean up partial read setup

[OllieinCanada]

Fixes #424

Summary

Preserve setup failures from hf3fs_fuse.io.read_file() and clean up only the
resources that were successfully acquired.

Root cause

The cleanup block unconditionally referenced fd, ior, iov, and shm.
When file opening, native registration, shared-memory creation, iovec creation,
or ioring creation failed, an UnboundLocalError could replace the useful
original exception. Failures after shared-memory creation could also skip its
close() and unlink() calls.

Fix

• initialize each resource before entering the setup path
• track whether native file-descriptor registration completed
• deregister and close the descriptor only when applicable
• release iovec, ioring, and shared memory only after they exist

Successful read behavior and cleanup order remain unchanged. This also lets an
underlying make_iovec() error such as #291 surface correctly; it does not
attempt to solve #291's separate symlink failure.

Tests

A host-only test with a stubbed native binding covers:

• preserving an os.open() failure
• not deregistering a descriptor whose registration failed
• releasing the descriptor after shared-memory creation fails
• releasing the descriptor, registration, and shared memory after iovec setup fails
• releasing the descriptor, registration, and shared memory after ioring setup fails
• unchanged successful reads and cleanup

Validation:

python -m pytest tests/fuse/test_hf3fs_fuse_io.py -q
6 passed in 0.81s

python -m compileall -q hf3fs_fuse tests/fuse/test_hf3fs_fuse_io.py
git diff --check

No 3FS mount, RDMA device, external service, or new dependency is required for
the regression test. The full deployment-dependent 3FS suite was not run in
this environment.